There has been proposed a tapping screw which is fastened to a prepared hole in a subject member while cutting a female thread in the subject by screwing the threads on the tapping screw into the prepared hole in the subject. In the case of screwing the tapping screw into the subject particularly made of resin, there is a demand of making a screwing torque as small as possible, and making a torque (a strip torque) by which a female thread portion in a peripheral surface of the prepared hole of the subject is broken as large as possible. Accordingly, in order to satisfy the demand, there have been proposed tapping screws having various shapes. For example, in a tapping screw shown in Japanese Unexamined Utility Model Publication No. 53-83958, an angle of a thread ridge is set to be equal to or more than 40 degrees and equal to or less than 50 degrees, and the thread ridge is provided with a plurality of notches (grooves), thereby lowering the screwing torque and preventing the strip torque from being lowered.
However, in the prior art as mentioned above, since a top portion and a bottom portion in the thread portion of the tapping screw are connected by a linear joint portion, there has been a problem that an excess thickness in the peripheral surface of the prepared hole, which is generated by deforming the subject using the thread ridge at a time of screwing the tapping screw into the prepared hole of the subject, is hard to flow into the bottom portion of the thread portion. If the excess thickness insufficiently flows into the bottom portion of the thread portion, a fastening state between the tapping screw and the subject becomes unstable, so that there is a possibility that the female thread portion in the peripheral surface of the prepared hole is broken, even by a small torque. Further, since a magnitude of the angle of the thread ridge is not that different from the angle of the thread ridge of the general screw, it is impossible to greatly lower the screwing torque, and it is impossible to reduce a radial component of a force applied to the female thread portion in the peripheral surface of the prepared hole from a slant face of the thread ridge of the thread portion, that is, a force for expanding the subject member in a radial direction, so that there is a problem that the subject may be expanded in the radial direction and be cracked.
Accordingly, an object of the present invention is to provide a tapping screw which can solve the defects in the prior art mentioned above, can make a screwing torque into a subject member small, and does not easily break a female screw thread in a peripheral surface of a prepared hole of the subject member.